Dr Heather Massey is a swimmer and a researcher at the Extreme Environments Laboratory at the University of Portsmouth. Here she gives a beginners guide to the adaptation response that takes place in channel and ice swimmers, meaning they can survive water temperatures which would kill the unitiated
The secret to acclimatising to cold water is just to swim in it, often – at least once a week, and preferably two or three, gradually extending the time that you stay in the water. Get out if you are not comfortable, and don’t set time goals for staying in the water.
It is easier to start your swimming career in summer when the water temperature is at 16°C and above, and then keep on swimming as the temperature drops. However, it not essential to swim in the cold water during the autumn and winter. The lakes, rivers and the sea can get very cold.
Going in to very cold water (typically less than 10oC) can cause numbness and pain, particularly in the extremities, such as the hands and feet. Neoprene socks and gloves can help protect your hands and feet.
The first thing that happens when you get into cold water is the cold shock response – the initial gasp, rapid breathing and increased heart rate and blood pressure. Swimmers can habituate this response very fast; as few as 5 or 6 three minute immersions where the whole body (not the head) are immersed in cold water will halve the cold shock response. Research shows that these dulled response are conserved for a period of time: if you miss a couple of weeks in cold water, you don’t start all over again. Half of this cold water shock reduction is present 14 months following the initial batch of cold water immersions.
Wetsuits do not remove the cold shock response when you get in: as it fills with cold water, the wearer will still experience the cold shock response. It’s after this moment that the benefit kicks in: the layer of water trapped between the skin and the wetsuit warms up.
The main source of adaptation that humans have to cold environments is our brain. We use this to create technologies such as shelter, clothing and heat sources. The body does have some limited capacity to adapt to the cold. But we must warn about becoming too cold in pursuit of cold adaptation. Repeated cold water swimming exposures that reduce your deep body temperature are the stimulus for this adaptive process. There are several other adaptive processes which cold exposure can cause, including a metabolic adaptation that involves increasing the shivering response to generate more heat from shivering. However, the predominant adaptive process in cold water swimmers is a combination of insulative and hypothermic adaptations.
The insulative adaptation indicates that the vessels supplying blood to the skin are reduced in size, limiting blood flow to the surface of the skin (vasoconstriction) and repeated exposure may stimulate a rapid vasoconstrictor response. This will ensure that more of the heat in the blood supply is moved the deep tissues of the body leaving the skin, hands and feet to cool to a greater extent, but insulating the core from cooling. This greater sensitivity to cold may also mean that it takes longer to rewarm your hands and feet compared to your friends who have not been cold exposed, as the blood vessels are slow to open and blood supply return to the fingers and toes.
Shivering is stimulated to increase deep body temperature when a reduction in deep body temperature occurs. It works by involuntary contraction of the muscles to generate heat and also makes movements like swimming difficult to coordinate. The hypothermic adaptation reduces the amount of shivering that occurs as the deep body temperature is reduced. By repeated bouts of swimming in cold water that reduces the deep body temperature this will reduce the shivering response. This has the effect of enabling the swimmer to coordinate movement, but at the expense of greater deep body cooling compared to someone who is shivering. The reduction in the shivering response is temperature specific, if deep body temperature cools beyond the normal fall in deep body temperature the shivering response is stimulated again.
The important point to note is that the deep body temperature at which shivering occurs in someone who is adapted is closer to the state of clinical hypothermia (core body temperature is 35°C) than non-adapted individuals. Therefore, those who are cold-adapted need to to ‘listen to their own body’ rather than the banter on the waters edge. If they are starting to shiver, they must get out of the water and start the process of rewarming.
The duration of time in the water is not only a reflection of the state of adaptation, but a combination of factors which relate to each individual including their mass, fatness, fitness, feeding regime and how long the person is stationary for. Recent technology and research suggests the existence of Brown Adipose Tissue (BAT) in adult humans; it was previously thought that this type of fat was only present in babies. However, it has now been found to exist in some individuals who are cold exposed, but the extent this contributes to thermoregulation when cold water swimming has not been established. The studies which have looked at cold water adaptation indicate that the predominate adaptive response is that of a hypothermic-insulatative adaptation, rather than a metabolic adaptation from an increase in heat production which would occur if BAT were activated.
Our core body temperature is normally stable but fluctuates (between 36.5 and 37.5oC) depending on the time of day, the environmental temperature, our clothing level, exercise, body fatness and what we have eaten. It is a balancing act between factors that keep it warm and those that cool.
Clinical hypothermia is considered a core body temperature of 35oC, so there is a small temperature gap between our body functioning normally and being in a serious medical condition.
Most people will feel the effects of hypothermia well before their core body temperature cools to 35oC. These signs include mental confusion, slurred speech and poor movement coordination. It is recommended that if you meet swimmers in this state or feel these yourself that they/you are evacuated from the water into shelter and slow rewarming is undertaken by dressing in dry clothes and lots of shivering.
Once you exit the water, you continue to cool for approximately 20-30 minutes. This means that your deep body temperature will be cooler 20-30 minutes after your swim than your were when you got out of the water. In other words, warming up immediately after your swim is vital.
The safest way to rewarm is to:
Keep yourself and others safe by waiting until you have warmed up before driving.
Want to find out more? Browse The Outdoor Swimming Society’s Survive section, particularly our other features on Cold.